I previously subscribed to Sirius and XM. I now subscribe to XM. These are my opinions, since we already have an insane XM vs. Sirius thread in Entertainment.
The music on both services are hit or miss. XM still has "the most" commercial free music (69 channels). 4 channels are controlled by Clear Channel and they have chosen (against XM's word) to air advertisements. Sirius plays the more popular hits while XM goes a bit deeper in the playlists. Sirius may still have the better rock channels, although XM's made some strides in Liquid Metal and Big Tracks. The decade channels are what they are...although Sirius technically doesn't have 90's channel.
For sports and talk, it's purely a matter of preference. If you want to join the Howard "revolution"...or whatever he's calling it now...then Sirius is your answer. If you want NFL, NBA, or English soccer action, go with Sirius. XM will have hockey for the next 9 years...8 which are exclusive. They also have MLB for the next 7 years. XM also has Opie and Anthony and Ron and Fez, which contrary to Sirius zombies, are actually doing well on XM and appear to be getting their own channels.
Technology clearly goes to XM. When I had Sirius, I could use my receiver as a space heater. The chipset just wasn't up to par and a battery drain. I hear it's more improved in the upcoming receivers, but we'll see. Sirius is making their strides in improving technology...but XM is already working on their 3rd generation portable now while Sirius just released their first.
Reception and quality was what sold me on XM. You can have a handful of satellites hovering all over North America, but you won't get signal around mountains and in cities. Both companies need to rely on a strong repeater network on the ground that will bounce signals around buildings, and sometimes through buildings. XM spent millions on an extensive repeater network (around 1,000 repeaters), Sirius spent it on Howard. Sirius' signal (back in early 2005) dropped frequently for me in Raleigh, NC. And I'd have the antenna aimed in the open air...and I'd have at most one bar. I only hope it's improved, considering their first true portable unit is about to hit the market.
All in all, it's a matter of preference. If you need Howard and the NFL and NBA and Martha Stewart, go with Sirius. If you want the NHL, MLB, O&A, and have a soft spot for Oprah...go with XM. You can adjust to the music.